Editing OpenVMS (section)

===Port to Alpha===
{{See also|DEC PRISM#Legacy|DEC Alpha#History}}
[[Image:dec-vms-vernon.png|thumb|"Vernon the Shark" logo for OpenVMS<ref>{{cite web|url=http://www.vaxination.ca/vms/shark/history.html|title=History of the Vernon the VMS shark|website=vaxination.ca|access-date=January 24, 2021}}</ref>]]
During the 1980s, DEC planned to replace the VAX platform and the VMS operating system with the [[DEC PRISM|PRISM]] architecture and the [[DEC MICA|MICA]] operating system.<ref>{{cite web|url=http://www.bitsavers.org/pdf/dec/prism/memos/880530_Cutler_PRISM_vs_MIPS.pdf|title=DECwest/SDT Agenda|author=Dave Cutler|date=May 30, 1988|website=bitsavers.org}}</ref> When these projects were cancelled in 1988, a team was set up to design new VAX/VMS systems of comparable performance to [[RISC]]-based Unix systems.<ref name="supnik-alpha">{{cite web|url=http://simh.trailing-edge.com/semi/ev4.html|title=EV-4 (1992)|date=February 24, 2008}}</ref> After a number of failed attempts to design a faster VAX-compatible processor, the group demonstrated the feasibility of [[porting]] VMS and its applications to a RISC architecture based on PRISM.<ref name="ieee-alpha" /> This led to the creation of the [[DEC Alpha|Alpha]] architecture.<ref>{{cite web|url=https://dspace.mit.edu/bitstream/handle/1721.1/48380/managingtechnolo00katz.pdf|date=April 1993|title=Managing Technological Leaps: A study of DEC's Alpha Design Team}}</ref> The project to port VMS to Alpha began in 1989, and first booted on a prototype [[Alpha 21064#History|Alpha EV3]]-based ''Alpha Demonstration Unit'' in early 1991.<ref name="ieee-alpha">{{cite journal|title=How DEC developed Alpha|date=July 1992|doi=10.1109/6.144508|last1=Comerford|first1=R.|journal=IEEE Spectrum|volume=29|issue=7|pages=26–31}}</ref><ref>{{cite journal|last1=Supnik|first1=Robert M.|title=Digital's Alpha project|journal=Communications of the ACM|volume=36|issue=2|year=1993|pages=30–32|issn=0001-0782|doi=10.1145/151220.151223|s2cid=32694010|doi-access=free}}</ref>

The main challenge in porting VMS to a new architecture was that VMS and the VAX were designed together, meaning that VMS was dependent on certain details of the VAX architecture.<ref name="ia64-port">{{cite journal|url=http://www.decus.de/events/alphamigration/vortraege/porting_openvms_to_integrity.pdf|title=Porting OpenVMS to HP Integrity Servers|author=Clair Grant|journal=OpenVMS Technical Journal|volume=6|date=June 2005}}</ref> Furthermore, a significant amount of the VMS kernel, layered products, and customer-developed applications were implemented in [[VAX MACRO]] assembly code.<ref name="vms-20-years" /> Some of the changes needed to decouple VMS from the VAX architecture included the creation of the ''MACRO-32'' compiler, which treated VAX MACRO as a [[high-level language]], and compiled it to Alpha [[object code]],<ref name="axp-port">{{cite journal|url=https://vmssoftware.com/docs/dtj-v04-04-1992.pdf|title=Porting OpenVMS from VAX to Alpha AXP|author1=Nancy P. Kronenberg|author2=Thomas R. Benson|author3=Wayne M. Cardoza|author4=Ravindran Jagannathan|author5=Benjamin J. Thomas III|journal=Digital Technical Journal|volume=4|issue=4|pages=111–120|year=1992|access-date=April 27, 2024}}</ref> and the emulation of certain low-level details of the VAX architecture in [[PALcode]], such as [[interrupt handling]] and atomic queue instructions.

The VMS port to Alpha resulted in the creation of two separate codebases: one for [[VAX]], and another for Alpha.<ref name="wizard-vms-source">{{cite web|url=http://h41379.www4.hpe.com/wizard/wiz_2993.html|title=Access to OpenVMS Source Code?|work=HP OpenVMS Systems ask the wizard|archive-url=https://web.archive.org/web/20171028043200/http://h41379.www4.hpe.com/wizard/wiz_2993.html|archive-date=October 28, 2017|date=September 2, 1999}}</ref> The Alpha code library was based on a snapshot of the VAX/VMS code base circa V5.4-2.<ref name="OpenVMS VAX and Alpha Compatibility" /> 1992 saw the release of the first version of OpenVMS for [[Alpha AXP]] systems, designated ''OpenVMS AXP V1.0''. In 1994, with the release of OpenVMS V6.1, feature (and version number) parity between the VAX and Alpha variants was achieved; this was the so-called Functional Equivalence release.<ref name="OpenVMS VAX and Alpha Compatibility">{{cite web|url=https://support.hpe.com/hpsc/doc/public/display?docId=emr_na-c04623258|title=OpenVMS Compatibility Between VAX and Alpha|date=May 1995|publisher=Digital Equipment Corporation|access-date=October 22, 2021}}</ref> The decision to use the 1.x version numbering stream for the pre-production quality releases of OpenVMS AXP confused some customers, and was not repeated in the subsequent ports of OpenVMS to new platforms.<ref name="ia64-port" />

When VMS was ported to Alpha, it was initially left as a 32-bit only operating system.<ref name="axp-port" /> This was done to ensure backwards compatibility with software written for the 32-bit VAX. 64-bit addressing was first added for Alpha in the V7.0 release.<ref>{{cite journal|url=https://vmssoftware.com/docs/dtj-v08-02-1996.pdf|title=Extending OpenVMS for 64-bit Addressable Virtual Memory|journal=Digital Technical Journal|volume=8|issue=2|date=1996|pages=57–71|s2cid=9618620}}</ref> In order to allow 64-bit code to interoperate with older 32-bit code, OpenVMS does not create a distinction between 32-bit and 64-bit executables, but instead allows for both 32-bit and 64-bit pointers to be used within the same code.<ref>{{cite journal|url=https://pdfs.semanticscholar.org/8d96/bbba019c43e80f059825311b67c975a0fede.pdf|archive-url=https://web.archive.org/web/20200219001633/https://pdfs.semanticscholar.org/8d96/bbba019c43e80f059825311b67c975a0fede.pdf|url-status=dead|archive-date=February 19, 2020|title=The OpenVMS Mixed Pointer Size Environment|journal=Digital Technical Journal|volume=8|issue=2|date=1996|pages=72–82|s2cid=14874367}}</ref> This is known as mixed pointer support. The 64-bit OpenVMS Alpha releases support a maximum virtual address space size of 8TiB (a 43-bit address space), which is the maximum supported by the [[Alpha 21064]] and [[Alpha 21164]].<ref name="prog-concepts-i">{{cite web|url=https://vmssoftware.com/docs/VSI_PROGRAM_CONCEPTS_VOL_I.pdf|title=VSI OpenVMS Programming Concepts Manual, Vol. 1|date=April 2020|access-date=October 7, 2020|publisher=VSI}}</ref>

One of the more noteworthy Alpha-only features of OpenVMS was ''OpenVMS Galaxy'', which allowed the [[Logical partition|partitioning]] of a single SMP server to run multiple instances of OpenVMS. Galaxy supported dynamic resource allocation to running partitions, and the ability to share memory between partitions.<ref name="galaxy-docs">{{cite web|url=https://support.hpe.com/hpesc/public/docDisplay?docId=emr_na-c04621422|title=HP OpenVMS Alpha Partitioning and Galaxy Guide|date=September 2003|publisher=HP|access-date=October 22, 2021}}</ref><ref>{{cite web|url=https://www.arnnet.com.au/article/111492/compaq_details_strategy_openvms/|title=Compaq details strategy for OpenVMS|date=October 14, 1998|access-date=January 14, 2021|author=James Niccolai|publisher=Australian Reseller News|archive-date=April 4, 2023|archive-url=https://web.archive.org/web/20230404095822/https://www.arnnet.com.au/article/111492/compaq_details_strategy_openvms/|url-status=dead}}</ref>